The present invention relates generally to bowed roll assemblies, and more particularly to an apparatus and system for adjusting a bowed roll assembly to alter critical speeds and inhibit harmonic vibration during operation of the assembly.
A bowed roll is a banana shaped, rubber covered or segmented metal roller used to remove wrinkles in a continuously moving web of media such as paper, foil, plastic film or the like. The bowed roll is comprised of a series of rotating segments (or spools), fitted with bearings, along a curved stationary axle.
The bowed roll is normally installed with the bow oriented more or less in the downstream direction. Under this condition, the incoming web contacts the roll surface on the backside of the bow. The web then remains in contact with the roll surface for 25-to-35 degrees of rotation before separating from the roll surface on the front-side of the bow. Since the roll is bowed, it stands to reason that the face width of the roll (measured along the roll""s surface) increases gradually from the back-side to the front-side. It is this xe2x80x9cwideningxe2x80x9d of the roll face that induces lateral tension (or stretch) in the web while it is in contact with the roll surface to reduce or remove wrinkles.
An example of a prior art bowed roll assembly 10 is depicted in FIG. 1. The bowed roll is comprised of a series of rotating segments or spools along a curved stationary axle (FIG. 3). The individual spools, each supported by a bearing centered within the spool, are placed along the axle and spaced so they turn freely yet in unison with each other. Referring to FIG. 1, the bowed roll includes a central axle 12 and a centerline 14, and is typically supported on each end with self-aligning mounting brackets 16. The position of the bow is adjusted by rotating the axle 12 in the self-aligning brackets 16. By changing the bow position, the amount of lateral stretch or tension in either the center or edges of the web can be controlled. In addition, the lateral tension induced by the bowed roll can also be used to aid in separating a single web which has been slit in two or more webs to prevent interweaving during winding operations or the like. Bowed rolls find numerous applications above and beyond the examples cited here.
At predictable rotational speeds, bowed rolls (as is the case with all rotating machinery) are subject to a multitude of harmonic vibration modes, which can occur during normal operation depending upon the construction of the roll, the materials used in its construction, and the process speed. The speeds at which the phenomenon of harmonic vibration occurs is often referred to as the xe2x80x9cwhirlingxe2x80x9d or xe2x80x9ccriticalxe2x80x9d speeds. The critical speed of a roll is essentially the rotational speed equivalent to the roll""s natural frequency of vibration. If a roll is operating at these speeds, oscillations may occur that can damage a forming sheet, among other things. This phenomenon is well known to those engaged in the design of shafts, rollers and other rotating machinery.
The first three critical speeds (xe2x80x9ccriticalsxe2x80x9d) are of great significance in that their related amplitudes of oscillation are great enough to either disturb the web handling process and/or result in damage to the spool bearing raceways and rolling elements. FIGS. 2A-2C illustrate the harmonic vibration mode shapes for a bowed roll centerline when operating at the first three critical speeds, and show the dynamic shape of the bowed roll centerline 14a, 14b, 14c at the first, second and third critical speeds, respectively. Bowed rolls have been proven to experience premature spool bearing failures when the roll is operated at these criticals for significant periods of time.
Referring to FIG. 3, an embodiment of a prior art bowed roll construction is shown. In the current state of the art/science of bowed roll construction, a series of spool assemblies 20 comprising a rotating segment (or spool) 22 fitted with bearings 24, are positioned along a curved stationary axle 12. These spool assemblies 20 are positioned so as to be in near proximity to each other and are coupled together individually by means of elastomer couplings 28. In the case of rubber-cover rolls, an elastomer covering or sleeve 28 (as shown) is fitted over the entire series of spool assemblies.
As depicted, the spool assemblies 20 are held in near proximity by means of annular spacers 30, which are fitted over the curved axle 12 (with the axle having an annular cross section) with their ends abutting the inner races 32 of the spool bearings 24. The entire series of spool assemblies 20 and spacers 30 are held in position along the bowed roll axle 12 by means of set collars 34 affixed to the axle 12 at either end 40a,b of the assembly 10.
In another embodiment of a bowed roll assembly, designated generally with the numeral 38xe2x80x2 and shown in FIGS. 4 and 5, the ends 40axe2x80x2, 40bxe2x80x2 of the axle 12xe2x80x2 comprise threads 46xe2x80x2 to which large reinforcing nuts 48axe2x80x2, 48bxe2x80x2 are affixed to the axle ends. Each end 40axe2x80x2, 40bxe2x80x2 of the axle 12xe2x80x2 is mounted in a roll supporting bracket 42xe2x80x2 that includes a ball clamp 44xe2x80x2. In this instance, a predetermined torque is applied to the nuts, which in turn loads the spool bearing inner races 32xe2x80x2 and annular spacers 30xe2x80x2 in compression, which induces an amount of tension in the axle and increases the stiffness of the roll. In the manufacture of bowed rolls, this technique is referred to as xe2x80x9creinforcing,xe2x80x9d and the assembly 38xe2x80x2 can be referred to as a reinforced bowed roll assembly. A reinforced roll is similar to a standard bowed roll (FIG. 3) except with a reinforced axle. As shown in FIG. 5, the ends 36axe2x80x2, 36bxe2x80x2 of the assembly 38xe2x80x2 include an end cap 50xe2x80x2 and an end shield 52xe2x80x2.
It is well known to those versed in the art/science of rotating machinery that the critical speeds for rollers, shafts, and the like, occur as a function of the roller""s mass and stiffness. The step of reinforcing a bowed roll increases the roll""s stiffness, which in turn alters the rotational speeds at which the roll""s xe2x80x9ccriticalsxe2x80x9d occur. By changing the torque applied to the axle nuts 48xe2x80x2, one can tune (i.e., increase, decrease, shift) the criticals to speeds out of the range of the operating speed of the bowed roll assembly so the criticals will not be encountered during normal operation for a known process machine speed. However, the adjustment of the applied torque requires that the roll be taken out of service and partially disassembled. Hence, it is not practical to perform this procedure in the field. While the ability to tune the criticals for a roll is helpful in preventing harmonic vibration, it requires that the machine process speed be limited. This is often unacceptable in that it may limit the machine""s productivity.
The present invention provides an improved bowed roll assembly for use in machines for processing paper and other continuous web of flexible media, among other applications.
In one aspect, the invention provides a bowed roll assembly that can be adjusted during operation of the assembly to alter critical speeds to outside the range of the operational speed of the assembly. The bowed roll assembly generally comprises a non-rotating central axle having a first end and a second end, at least two tubular segments, each supported on a rolling bearing and rotatably mounted on the central axle, and a plurality of annular spaces mounted on the axle between the roller bearings to maintain the tubular segments in near proximity.
In one embodiment of a bowed roll assembly, the first end of the axle comprises a member mounted thereon for applying or loading pressure against the bearings and the spacers which can be varied to compress and uncompress the tubular segments along the axle and alter the stiffness of the roll assembly, and the roll assembly is connected to a member for detecting speed of the roll assembly and sending a signal of the speed to a signal processing device. When the speed of the roll assembly approaches a critical speed, the signal processing device sends a signal to the pressure applying member to vary the pressure against the bearings and spacers to sufficiently stiffen or unstiffen the roll assembly to inhibit harmonic vibration of the roll assembly and shift the critical speeds out of the range of the operating speed of the bowed roll assembly so the criticals will not be encountered during normal operation for a known process machine speed.
In another embodiment of the roll assembly, the signal processing device can be pre-programmed with critical speed values of the bowed roll assembly. When the signal processing device receives a signal of the rotational speed of the bowed roll assembly at or near a critical speed value, the signal processing device sends a signal to the pressure applying member to increase or decrease the pressure against the spacers to sufficiently stiffen or unstiffen (relax) the roll assembly to inhibit harmonic vibration of the roll assembly.
In another embodiment of the roll assembly, the assembly can comprise an accelerometer mounted on the roll assembly. During rotation of the roll assembly, an increase in the voltage output of the accelerometer relative to a voltage output set value indicates the rotational speed of the bowed roll assembly is at or near the critical speed value. Once the pressure of the pressure applying member has been adjusted, a decrease in the accelerometer voltage output relative to the voltage set value indicates an increase or decrease of the critical speed value.
In yet another embodiment of the roll assembly, the signal processing device comprises a programmable logic controller (PLC) functional for receiving and recording data of the accelerometer voltage output, the rotational speed of the roll assembly, and/or the pressure load applied by the pressure applying member. This enables the PLC to xe2x80x9clearnxe2x80x9d the critical speeds of the roll assembly associated with any given hydraulic pressure set point. When the rotational speed of the assembly the approaches that critical, the PLC signals for a change in the pressure of the pressure applying member to stiffen or unstiffen the assembly and alter the critical outside the range of the operational speed.
In another aspect, the invention provides a system for altering stiffness of a bowed roll assembly. The system comprises a bowed roll assembly comprising at least two spool assemblies mounted on an axle and held in near proximity by adjacently mounted annular spacers, a member for applying pressure against (compress or uncompress) the spacers and the spool assemblies to increase or decrease the stiffness of the bowed roll assembly that is mounted on one end of the axle, and a programmable logic controller (PLC) connected to the bowed roll assembly for receiving signals therefrom and connected to the pressure applying member such as a hydraulic ram, for transmitting signals thereto. The PLC can be connected to a mechanism for measuring a machine speed reference signal. In one embodiment, the PLC can comprise a microprocessor that is pre-programmed with critical speeds of the bowed roll assembly. The PLC is functional to send signals to the pressure applying member to alter the pressure load on the spacers/spool assemblies upon sensing a rotational speed at or near a pre-programmed critical speed value. In another embodiment, the system can comprise an accelerometer mounted within the axle of the bowed roll assembly that is connected to the PLC. Voltage output data of the accelerometer can be sent to the PLC to indicate the rotational speed of the bowed roll assembly relative to a critical speed value.
In another aspect, the invention provides methods of altering critical speeds of the roll assembly during operation to inhibit harmonic vibration. The bowed roll assembly generally comprises at least two spool assemblies mounted on an axle and held in near proximity by adjacently mounted annular spacers, and a member for applying pressure against the spacers to alter stiffness of the bowed roll assembly.
In an embodiment of a method according to the invention, rotational speed of the bowed roll assembly is measured, the rotational speed is compared to a pre-programmed critical speed value, and upon sensing the rotational speed at or about the pre-programmed critical speed, the pressure of a pressure applying member against the spacers is altered to increase or decrease the stiffness of the bowed roll assembly and alter the critical speed to a range outside of the roll rotational speed. The method can further comprise the step of pre-programming critical speeds into a PLC connected to the assembly.
In another embodiment of the method, the bowed roll assembly further comprises an accelerometer mounted on the roll assembly, and the method comprises the steps of measuring rotational speed of the roll assembly, measuring (and recording) voltage output of the accelerometer, and determining whether the rotational speed of the roll assembly is at or near a critical speed based on the voltage output of the accelerometer. When the rotational speed is at or near a critical speed, the pressure of the pressure applying member against the spacers is altered to increase or decrease the stiffness of the roll to alter the critical speed to a range outside of the roll rotational speed. The method can further include measuring the voltage output of the accelerometer to determine whether the critical speed has been so altered. A decrease in voltage output of the accelerometer indicates an increase or decrease in the critical speed.
In another embodiment of the method, wherein the bowed roll assembly further comprises an accelerometer mounted on the roll assembly, the method comprises the steps of measuring voltage output of the accelerometer to determine whether the rotational speed is at or near a critical speed value, wherein an accelerometer signal voltage higher than set voltage output value indicates the rotational speed to be at or about the critical speed; and upon sensing the rotational speed at or about the critical speed, increasing or decreasing the pressure of the pressure applying member against the spacers to alter the stiffness of the roll and the critical speed value to a range outside of the rotational speed of the roll. The method can further include measuring the voltage output of the accelerometer to determine whether the critical speed has been altered, as indicated by a decrease in the accelerometer voltage output which indicates an increase or decrease in the critical speed.
In yet another embodiment of the method, the bowed roll assembly includes a PLC and accelerometer, and the critical speeds of the roll assembly are xe2x80x9clearnedxe2x80x9d by the PLC which triggers changes to the pressure of the pressure applying member of the assembly to alter the roll""s stiffness and the critical speeds. The method comprises measuring the rotational speed of the roll assembly; measuring the voltage output of the accelerometer; determining if the voltage output of the accelerometer exceeds a set value whereby the rotational speed is at or about a critical speed value; when the accelerometer voltage output exceeds the set value, recording the voltage output and the rotational speed by the PLC as a value to indicate a critical speed; when the rotational speed is at or about a critical speed, altering the pressure of the pressure applying member against the spacers to alter the critical speed to a range outside of the rotational speed of the roll. The method further comprises measuring the voltage output of the accelerometer and the rotational speed of the roll during the continued operation of the assembly whereupon, when the voltage output and rotational speed reach the critical speed value that has been recorded by the PLC, altering the pressure of the pressure applying member to alter the critical speed outside the range of the rotational speed of the roll. The accelerometers mounted inside the bowed roll can then be used to verify that the critical speed has been altered according to a decrease in the voltage output level.